A review of the seismotectonics of the Makran Subduction Zone as a baseline for Tsunami Hazard Assessments
The Makran Accretionary Wedge (900 km across) is a consequence of northward subduction of the oceanic part of the Arabian Plate beneath the Lut and Afghan blocks in the northwestern Indian Ocean. It has a complicated tectonic setting as it is located at a triple junction with the Indian Plate. Thick sedimentary layers, a shallow angle of the subducting slab and a large width of the subduction zone, ca. 500–600 km from volcanic arc to active wedge front, are some of the foremost and distinctive characteristics of the Makran Subduction Zone (MSZ). The MSZ is likely divided into at least two segments: the west and the east possibly separated by a sinistral fault known as the Sonne Fault. A division is also inferred from seismicity as it is higher in the east when compared to the west. With the exception of a notable trench, all other characteristics of an accretionary prism observed in well-studied subduction zone can be identified or inferred in the Makran. Three long seismic profiles of the western Makran (200 km long each, with shot points interval of 20 km and receivers interval of 700 m) have recently been acquired. Using these datasets, improved structural/velocity models for the western Makran were developed. This review aims to contribute to achieving a better understanding of the seismotectonic setting and dynamics of the Makran Subduction Zone as it feeds to a refined understanding of the tsunami hazard in the region.
KeywordsMakran Tsunami Oman Sea Reflection data Subduction zone Tsunami Early Warring
A subduction zone along the Makran coasts that forms the boundary between the Arabian and Eurasian Plates was first noted by Stoneley (1974). Later, foundation work by Shearman (1977) and Farhoudi and Karig (1977) followed by others such as Page et al. (1979) used field surveys of the Iranian coastline, air photo analysis, and aerial reconnaissance, to confirm the existence of a subduction zone and build a tectonic model to characterize the modern Makran Subduction Zone (MSZ).
Thick sedimentary layers, a shallow angle of the subducting slab and a large width of the subduction zone, ca. 500–600 km from volcanic arc to active wedge front, are some of the distinctive features of the MSZ (Mokhtari et al. 2008; Mokhtari 2014; Heidarzadeh et al. 2008a, b; Smit et al. 2010). The MSZ appears to be divided into at least two segments: the west and the east possibly separated by a sinistral fault known as the Sonne Fault (Fig. 1). This was supported by Kukowski et al. (2000) who introduced a new boundary aligned with the Sonne strike-slip fault.
Additional work based on geomorphological and seismological observations onshore also suggest that the subduction zone is likely to consist of two different domains located east and west of the Sistan Suture Zone, a structure that is the continuation of the Sonne Fault system located offshore (Byrne et al. 1992; Kukowski et al. 2000; Okal and Synolakis 2008; Heidarzadeh et al. 2009).
Regarding seismicity, several tsunamigenic earthquake events have occurred in historical times in 1756 and 1851 in the east and 1483 in the west. It is immediately important to mention that the location of 1483 event has been associated with the Strait of Hormuz by several authors, notably Musson (2009) and Rajendran et al. (2013). The most recent large tsunami event occurred on 27 November 1945 associated with an earthquake of magnitude 8.1 in the east Makran. It caused about 4000 casualties along the Makran coast affecting Iran, Pakistan, Oman and the United Arab Emirates (Heck 1947).
Since 1945 both coastal population and economic growth surrounding the Makran region have been increasing very rapidly and the vulnerability of these communities underscores the necessity for the development of Tsunami Hazard Assessments as an essential part of the development process. There has been considerable effort put into tsunami modeling for the MSZ using mainly the 1945 tsunami (e.g., Heidarzadeh et al. 2008a, b; Okal and Synolakis 2008; Burbidge et al. 2009). Such models are used as the basis for tsunami Early Warning System and preparedness; however, there are issues with these models that are primarily concerned with the source input parameters. As such, there are major ongoing efforts being conducted to acquire data onshore and offshore to improve the understanding of geophysical and source parameters. Such efforts will likely assist in the formation of tsunami assessments that have higher accuracy.
In this paper, after reviewing the seismicity of the Makran region, the seismotectonic framework of the area is addressed. Thereafter, the seismic expression of the Makran offshore zone in the east and west will be compared and contrasted. Finally, a preliminary Tsunami Hazard Assessment is provided along with an outline for a Tsunami Early Warning System and future planned work.
The MSZ is unique due to its geological and seismological characteristics. It shows relatively low seismicity in comparison other subduction zones worldwide (Mokhtari et al. 2008; Heidarzadeh et al. 2008a, b). Despite the relatively low seismicity, several tsunamigenic events have been reported in the MSZ. Moderate magnitude earthquakes on the western section of the Makran were believed to have occurred in 1008 and 1483, the latter associated with the Strait of Hormuz and probably not on the subduction mega-thrust (Musson 2009; Rajendran et al. 2013). According to Murty and Bapat (1999), a tsunami was observed in the northern Indian Ocean on the southern coast of Iran from a local earthquake in 1008. Ambraseys and Melville (1982) discussed this event in more detail and reported that the earthquake and tsunami sunk some ships and killed a large number of people (there is no information on the actual numbers). It has been reported by Rastogi and Jaiswal (2006) that the epicenter of the earthquake was likely near 25.01 N and 60.01E in the western Makran Subduction Zone.
In contrast, several significant events have been reported along the eastern part of MSZ, including earthquakes in 1756, 1851 and 1945 (Fig. 1). The 1945 Mw 8.1 earthquake is by far the largest instrumentally recorded earthquake in the area (Byrne et al. 1992). The rate of seismicity in the westernmost of the 1945 earthquake ruptured zone is inferred to be higher than the east Makran (Quittmeyer and Jacob 1979). This area overlaps with the easternmost of the rupture zone of the 1851 earthquake and there is speculation about the recurrence of a future earthquake in this area (e.g., Byrne et al. 1992; Pararas-Carayannis 2006).
Lin et al. (2015) inferred the distribution of inter-seismic coupling on the eastern Makran mega-thrust from 2003 to 2010. They found high inter-seismic coupling (i.e., the mega-thrust does not slip and elastic strain accumulates) in the central section of eastern Makran, where the 1945 earthquake occurred, while lower coupling coincides spatially with the subduction of the Sonne Fault Zone. The inferred accumulation of elastic strain since the 1945 earthquake may imply the future occurrence of magnitude 7+ earthquakes and could not exclude the possibility of a multi-segment rupture that would likely exceed Mw 8. These data suggest high fault coupling in the area where a magnitude 8.1 tsunamigenic earthquake ruptured 73 years ago. Lin et al. (2015) ruled out models where the interface is fully creeping as well as models where coupling is homogeneously distributed down to 20–30 km depth.
Unlike other subduction zones, globally there is no trench in the MSZ (Schluter et al. 2002; Mokhtari et al. 2008). Oceanic trenches are the direct manifestation of underthrusting oceanic lithosphere and are developed on the oceanic side of subduction zones with depths of about 2–4 km below the surrounding Ocean Floor (Kearey and Vine 1996).
Seismic profiles also document large normal fault systems near the coast, which are also found on land in the Coastal plain area (Dolati 2010). Volcanic arcs were thought to form approximately 100 km above the contour of subducting slabs (Davis et al. 1983) but significant variations have been found (e.g., England et al. 2004; Syracuse and Abers 2006). This is probably due to the dependence of thermal structure on convergence rate, plate dip and age (England and Wilkins 2004). Penney et al. (2017) proposed that the hinge line lies just south of the volcanic arc in the Makran. This arc is composed of three main volcanic centers (yellow stars in Fig. 4), which have chemical signatures indicative of subduction volcanism (Biabangard and Moradian 2008; Nicholson et al. 2010; Saadat and Stern 2011). Earthquakes deeper than the proposed hinge line have only been recorded in the east with the exception of an isolated deep event in the far west, reported by Maggi et al. (2000). However, one of the normal-faulting earthquakes occurred almost directly beneath Bazman volcano, at a depth of 74 km (Jacob and Quittmeyer 1979). This suggests the subduction interface is not deeper than ~ 80 km under this part of the volcanic arc, allowing for ~ 5 km possible error in the earthquake depth. For the subduction interface to reach a depth of ~ 100 km by the latitudes of Taftan and Sultan (at 62° E) it would need to significantly increase its dip north of the line of normal-faulting earthquakes. As the earthquakes likely occur within the descending slab and are related to normal-faulting, they provide an upper limit on the average dip of the subduction interface in the region.
Penney et al. (2017) suggest overall convergence rates of 20.4 mm/year and 32.6 mm/year in the west and east of 59.5 E, respectively, based on surface velocity from a coupling model with a locking depth of 30 km, mean dip of 11°. However, earlier studies that employed a network of 27 GPS (Global Position System) in Iran and Northern Oman reveal that the subduction rate at the Makran zone is about 19.5–27 mm/year (Vernant et al. 2004). Another study based on the GPS measurement by Masson et al. (2005) inferred a subduction rate of about 18 mm/year. When compared with other subduction zones in other parts of the world, this value is considerably lower, for example, in Aleutians, it is about 60–70 mm/year; in Sumatra, about 70 mm/year; in Kamchatka, about 75–78 mm/year; in Chile, about 80 mm/year; in Japan, about 90 mm/year and in Tonga, it is about 165 mm/year (Gorbatov and Kostoglodov 1997).
Seismic expression of the subduction zone
As it can be seen in the above-mentioned figures, despite the difference in the rate of seismicity in east and west Makran, the main structural elements are very similar. For example, both areas have a northward dipping thrust pack that is easily detectable within the accretionary prism. Also, southward converging reflectors beneath the abyssal plain are identified representing north side dipping geometry of the underlying oceanic crust.
Recent seismic data and essence of hazard assessment
In an effective Tsunami Hazard Assessment in any particular region, the historical records of tsunami occurrences and the potential for tsunami generation should be evaluated considering the potential seismic parameters of the source.
Based on tsunami modeling and allowing for the uncertainty of the seismic parameters of 1945 earthquake, in the Makran region using different earthquake scenarios it has been found that any probable tsunami in the Makran will likely hit the nearest coastline within about 15 to 20 min. This suggests an element of urgency for the establishment of a clear education system for what coastal communities should do if they feel and earthquake along with parallel efforts to establish an effective tsunami early warning system in the region. Such earthquakes and associated tsunamis present a significant hazard to populations around the Arabian Sea.
Conclusions and recommendations
The evolution and deformation history of the accretionary complex is the result of a continuous process, not a series of separate events, so it is important to study both onshore and offshore Makran.
The seismicity varies significantly between east and west Makran, but, in general, both sides have the capability of generating large earthquakes 8+ (Mw), which may result in large near-field tsunamis.
In an effort for tsunami risk reduction, improving our knowledge of the past earthquakes and tsunami occurrence in the region is highly required; therefore, in this respect, more details on Paleo-tsunami studies should be conducted in the future. It is vital to generate more realistic Probabilistic Tsunami Hazard Assessments (PTHA) for the region and the affected countries. Improved hazard assessments may allow the optimization of Early Warning Systems and the development of realistic preparedness strategies in national and local levels. Such efforts should be done in parallel with an education program for what to do when coastal communities feel an earthquake.
The Mud Volcanoes, both onshore and offshore, can be considered as another factor of risk enhancement. Mapping these features and understanding their generation can also play a vital role in tsunami risk reduction in the region.
The acquired active seismic refraction and wide-angle reflection onshore profiles have shown a turning point in better understanding of seismotectonics in the area; it is important to complement these data with long seismic profiles offshore for better understanding of the source location and its parameters.
This research was supported by the National Center for Earthquake Prediction, International Institute of Earthquake Engineering and Seismology (IIEES), under the Project: Makran study which is in cooperation with the Institute of Geoscience in Germany (GFZ). The major financial support for active seismic data acquisition was supported by Iranian Planning and Budget Organization (PBO).
MM conducted the data analysis and solutions, and wrote the major portion of the manuscript. AAA, LM and MR have done the seismotectonic part and helped with figures and writing of the manuscript. All authors read and approved the final manuscript.
Funding was provided by Makran National Mega Project at IIEES (Grant no. 345347).
The authors declare that they have no competing interests.
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